1. Field of the Invention
This invention relates to the sensing of methane in an atmosphere and has particular, though not of course exclusive, application to the remote sensing of methane at a facility such as a port or mine. The object of such sensing is to become aware when the amount of methane present is approaching an explosive or otherwise dangerous level.
2. Description of the Related Art
It is known to employ spectroscopic techniques to sense the presence of methane in an atmosphere. Past proposals have included the use of a He:nE laser to detect the strong 3.39 .mu.m absorption line (U.S. Pat. No. 3,998,557), an Er: YAG laser tuned at 1.645 .mu.m (Watkins et al, Rev. Sci Instrum (1981) 52(111), 1682), and an InGaAsP semiconductor laser to detect the v.sub.2 +2v.sub.3 absorption band around 1.33 .mu.m. (Chan et al, Applied Optics, (1983) 22(23), 3802). None of these proposals has proven entirely satisfactory for wide commercial application. The strong absorption lines of methane, e.g. the 3.39 .mu.m line, have not been suitable for remote sensing over considerable distances such as at a port facility or open cut mine as the absorption by nearer methane masks the position at a greater distance. The various lasers suggested suffer from one or more of several disadvantages including inadequate power, high cost and insufficient ruggedness.